This invention relates to a method of transmitting information, in which n-bit information words are converted into m-bit code words before transmission and said m-bit code words are re-converted into n-bit information words after transmission, and in which, for converting consecutive n-bit code words into m-bit information words with a limited maximum disparity .+-.d prior to transmission, where n, m and d are integers which comply with n&lt;m and d&lt;m, in such a way that the digital sum value taken over all the preceding code words at the beginning of a code word remains limited to a range which is bounded by a first and a second value, the following code word is selected, at least with respect to the polarity of the disparity, as a function of said digtal sum value over all the preceding code words so as to ensure that said following code word cannot cause an increase of the absolute value of said digital sum value, for which purpose a pair of code words is assigned to at least a first group of possible n-bit information words, the code words of said pair having opposite disparities with an absolute value d and being the bit-by bit inverse of one another for each associated information word.
The invention also relates to an encoding device for use in the method, for converting n-bit information words into m-bit code words and to a decoding device for use in the method, for converting m-bit code words into n-bit information words.
Such a method and such devices are known inter alia from GB-PS No. 1,540,617 and U.S. Pat. No. 4,387,364.
Such a conversion of n-bit information words into m-bit code words is employed in order to meet specific requirements imposed on the series of m-bit code words. This means that not all the possible combinations of m-bit code words in every possible sequence are allowed, so that the number of bits m is necessarily larger than the number of bits n of the associated information words. In the known method and devices, m may be even, or odd. If m is even the disparity 0 will occur in addition to the even disparities .+-.2, .+-.4 etc., and if m is odd the disparities .+-.1, .+-.3 etc. will occur. The maximum disparity is then .+-.m. This maximum disparity is limited (d&lt;m) to achieve a maximum code efficiency; raising the maximum disparity will result in a less-than-proportional increase of the number of possible code words, whereas the low-frequency content of the spectrum and the maximum number of successive ones or zeros (important for the clock generation) will increase substantially. The polarity is chosen as a function of the digital sum value over the preceding code words in order to obtain a d.c.-free transmission signal. This can be achieved in an advantageous manner by selecting for every information word, two code words which are the inverse of one another, so that only one of the two code words need be generated because the other word can be found by inversion.
Another important aspect is the generation of a decision level at the receiving end in order to decide whether a received bit is a logic 0 or a logic 1. This may be achieved by filtering the instantaneous digital-sum-value level. It is important that the time constant of the filter used for this purpose is as small as possible to enable rapid variations of the average digital-sum-value level to be followed. Therefore, it is essential to limit the amplitude of instantaneous digital-sum-value, variations may give rise to variations of said decision level (base-line wander). To this end limits may be imposed on the maximum excursion within the code words, for example by limiting the maximum instantaneous digital-sum-value to .+-.(d+2). This often means that there is a substantial surplus of permissible code words in comparison with the required number 2.sup.n. However, a reduction of this range to .+-.(d+1) results in an insufficient number of possible code words and an asymmetrical limitation to, for example, +(d+1) and -(d+2) makes no sense when the inversion principle is used because in that case all the pairs of code words of which one word is not within said limits will not conform, so that the number of possible code words is not larger than in the case of a limitation to the levels .+-.(d+1). The same applies to other limits, for example .+-.(d+3) in comparison with .+-.(d+2).